chore(ci): relink `python3` to `python3.9` on Homebrew
chore(ci): relink `python3` to `python3.9` on Homebrew

Implements a work-around for the compatibility issue between mozjs and
Homebrew's Python 3.10 formula.

servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs